I have an American Standard older system (34) where the blower constantly runs.I replaced the inducer unit earlier this year with no problems back in January.The blower runs constantly unless I manually flip the power switch at the unit itself. If I turn the furnace back on at the unit, the inducer blower kicks in, ignitor lights for exactly 45 seconds and then the blower kicks in which shuts the flame completely down? Then blower runs until I manually shut it off at the unit.I have now replaced the fan limit switch , which has not helped at all. Any thoughts?

"the inducer blower kicks in, ignitor lights for exactly 45 seconds and then the blower kicks in which shuts the flame completely down? Then blower runs until I manually shut it off at the unit."

When you say the "ignitor lights", do you mean the ignitor glows or the ignitor glows and then ignites the gas?

When the blower starts and "shuts the flame completely down" do you mean the gas valve closes (you can hear that) or do you mean the blower disturbs the flame pattern causing the ignition sensor to shut the flame down maybe 5 to 10 seconds after the blower starts? When the furnace starts, watch the main burner flame carefully. If you notice any flame 'dancing' or excess movement of the flames when the blower starts, you may have a crack in the heat exchanger.

What was the part number of the fan limit switch? What is the control board make and model number?

Ignitor glows and actually fires up.American Standard board ( D330937p01 Cnt 2184) and the exchanger was cleared as being unbelievably good back in January 2012. It was dye tested. Back in January it was a "short cycle" issue? The fan limit switch I replaced was L200F-30 and I did check the part number when it arrived and they both have P05 stamped on the label.

Regarding the flame not igniting. The ignition process is the inducer starts first. A vacuum sensor verifies the inducer fan is working properly and then the HSI will be powered, the gas valve will open and the gas ignite. A flame sensor monitors the gas flame and if there is no gas flame, the entire furnace is shut down.

The flame sensor is a rod that has a voltage applied for the purpose of creating a current flow through the actual flame. A gas flame will conduct electricity. The control board monitors the current flow and if the current is not adequate, because there is no flame, then the furnace is shut down.

The causes for an inadequate current flow can be a dirty sensor rod (clean with steel wool or emery cloth), improper positioning relative to the the flame and the burner (the current has to flow from the rod, thru the flame to the burner or other grounded metal surface of the furnace), or a poorly grounded furnace frame (check all 120 volt wiring back to the main circuit breaker box for good connections).

In your situation you can try and get the flame to stay lit. However the blower issue seems to indicate a bad control board. The part you replaced ( L200F-30) had nothing to do with the fan... it's a high limit safety to keep the furnace from overheating. Usually the fan switching is through a relay on the control board. All the issues you seem to have could all be explained by a flaky control board.

I know nothing of furnaces, but the tech last January did write a recommendation for an HSI, what is an HSI? and could this being fixed fix my entire problem? Is it a switch, a hose etc?And, the control board you reference is the Circuit Board? where all the wires are plugged into? I think I can get one on-line for $250.

HSI * Hot Surface Ignitor. It's the device that glows white hot and causes the gas to ignite. They are very delicate and if you want to do it yourself, read about Hot Surface Ignitors. One thing is not to touch the surface that glows.

The control board is the board the wires plug into including the wires from the thermostat. I would suspect the control board as bad (the fan relay) but if you decide to have a tech install the HSI he may be able to tell you for certain.

Look at the control board where the plug for the blower motor attaches. Close to that place you should see a small rectangular box, usually black. That could be the relay. Copy all the information off that relay and see if the part is even available.

If you can find one and know how to de-solder integrated circuit components, then replacing the relay is a good idea. Also check the board for any signs of cold solder joints (loose solder connections) and fried components, just to be sure there isn't more damage.

I am all for repairing these boards. It is simple but finding some components can be a deal-breaker.

Last thing!!!!!!They say it is a pressure switch and control board.Question? I have put a new inducer motor, flame sensor, ignitor and fan limit switch on the furnace.The company says they can sell me an entire new furnace for $4700 or replace heat exchanger (warranty), blower motor, pressure switch and control board for approx $2300.I would save approx $2400 and essentially have a new furnace? or would you advise on the new unit (Rheem)?

If you live in a cold climate then a more efficient system may be a far better way to go than repairing your old system. If you live in Florida, then repair the old system. Find a good installer and see what brands he/she recommends.